Polypropylene is a thermoplastic resin obtained by polymerizing propylene. Polypropylene fibers are produced by a well known melt spinning process, in which molten polymer is pumped through a die having a large number of small openings to produce a tow of continuous filaments. The filaments are cooled and drawn or elongated to increase tensile strength. A size or finish is usually applied to the filaments, followed by drying and tempering if required and optionally chopping into the desired length to provide bundles of fibers.
Since polypropylene has excellent characteristics such as a low density, a high strength, and an excellent resistance to heat and chemicals, polypropylene fibers are used in a wide variety of applications, such as textiles, packaging materials, containers, automobile components, etc.
European patent EP1812628 as well as equivalent US application US2013/0302608 disclose a process for producing melt-spun polypropylene multifilaments and yarns formed therefrom. The process involves forming melted polymeric composition requiring as an essential feature the presence of a nucleating agent, extrusion of the formed melted polymeric composition, quenching of the formed filaments in a liquid bath at a temperature of between 50° C. and 130° C. and drawing the filaments while heating at a temperature of between about 120° C. and about 150° C., wherein both the presence of the nucleating agent and the sufficiently long dwell time of the filament in the liquid bath ensure a sufficient degree of crystallization of the filament. The disclosed fibers are indeed highly crystalline, exhibiting more than about 80% crystallinity as measured using WAXS and SAXS techniques. It is further disclosed in EP1812628 (as well as in US2013/0302608) that these polypropylene fibers produced by this process can be utilized for forming ropes and woven or non-woven fabrics. While highly crystalline polypropylene fibers indeed may have their benefits in certain applications, such as for example for the production of textiles, a high crystallinity is in fact not an advantageous property for the production of fiber reinforced cement products. Indeed, it is known that polypropylene fibers having a high degree of crystallinity, typically exhibit a relatively low tenacity (i.e. a low fiber strength). For reinforcement of fiber cement products, however, the tenacity of the incorporated fibers is the main and essential property that contributes to the mechanical strength of the fiber cement end product. Therefore, the highly crystalline filaments and fibers as disclosed in EP1812628 (and equivalent US2013/0302608) are not suitable for the production of fiber reinforced cement products, especially given the fact that in every country all over the world these products are subject to strict national rules and regulations, stating the minimum requirements for mechanical strength of such products in order to ensure both safety for construction workers as well as performant products for the end users.
Despite the fact that several further variants to the production methods disclosed in EP1812628 have been developed, the polypropylene fibers produced by these known processes show an insufficient performance to be suitable for applications where a very high fiber strength is demanded, such as in fiber cement applications. Furthermore, while different research groups have tried to develop alternative methods to the known industrial process(es) in order to obtain polypropylene fibers with at least a higher fiber strength, such methods proved not to be feasible at industrial scale. Indeed, up-scaling from laboratory to industrial scale presents different challenges with regard to both technical and economical feasibility. Also, certain lab scale methods lead to inconsistent and varying filament or fiber qualities when performed even only at semi-industrial scale.